Essay on Clays and Pottery

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Clays and Pottery

Ceramicists, working either on a wheel or building by hand, define three main classes of clay bodies or mixtures: earthenware, stoneware, and porcelain. These are divided on the basis of firing temperatures and the character (hardness, vitrification and durability) of the final material. Each clay body is composed of a mixture of clay minerals and other materials such as sand or fine gravel and "fluxing" agents which affect the color and texture when fired.

Instead of discussing clays solely in terms of their chemical formulae, determined by x-ray diffraction, potters group clays into classes based on more general properties of the entire clay body, such as texture and color. One distinction potters make is between
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Within the secondary (sedimentary) clays, potters classify ball clay, fire clay, and low grade fire clay (or stoneware clay). Ball clays are highly plastic, and are added to clay bodies in order to increase plasticity. They vary widely, and their color, usually dark or reddish, is often a function of the amount of organic matter and/or iron in the clay body. Ball clays are more fusible than refractive and also tend to shrink appreciably during drying and firing. Since, according to Conrad, "the plasticity of clay is determined by particle shape and size, type of clay mineral, and the relative presence in the clay of soluble salts, absorbed ions, and organic matter (Conrad p. 9)," it remains unclear whether the type of clay minerals in the body are the defining factor in classifying a ball clay. I hypothesize based on the overall properties of the body that the clays are in the smectite class, allowing for a large shrink swell differential resulting in a high plasticity. Given that plasticity within a clay body is reliant on several factors, my assumption here is based on the idea that intraparticle properties of swelling during water absorbtion affects plasticity. The increase in the distance between the tetrahedral and octahedral layers with the absorbtion of water, would weaken the Van der Waals forces between the layers and allow for more slip. The question remains

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